Boeing XB-39 Superfortress image - Aircraft Lovers Group

The Boeing XB-39 Superfortress was a United States prototype bomber aircraft, a single example of the B-29 Superfortress converted to fly with alternative powerplants. It was intended to demonstrate that the B-29 could still be put into service even if the first choice of engine, the air-cooled Wright R-3350 radial engine, ran into development or production difficulties.

Starting life as the first YB-29 delivered to the United States Army Air Forces, it was sent in November 1943 to the the Fisher Body Aircraft Development Section of General Motors to be converted to use Allison V-3420-17 liquid-cooled X24 (twin-V12, common crankcase) inline engines. Testing on it began in early 1944. Further development of the engine and the aircraft was delayed by a series of changes in the planned turbosuperchargers as the originally specified GE Type CM-2 two-stage turbosupercharger became unavailable due to demands on GE's production of its other turbosuperchargers. Other turbosuperchargers were considered, but the end result was that the first flights of the B-39 had to be made without any turbosuperchargers at all. Now retired, prototypes scrapped.